1. Field of the Invention
The present invention relates generally to the forming of precision-molded articles such as hard molds for plastic injection molding or die casting from mixes of refractory or laser sinterable particles in heat fugitive organic binders. In particular, it relates to an improved formulation for an enhancer that improves the surface appearance and quality of the molded refractory or laser sintered article.
2. Description of the Prior Art
Prior processes for refractory molding included the two-step use of a furnace process that required a sintering step followed by a bronze metal infiltration step. U.S. Pat. No. 4,554,218 describes one such process. In the prior process, a binder is solely burned out of the parts after which the parts or articles are sintered at an elevated temperature and then the furnace in which the parts are sintered is cooled. The sintered parts are then removed and sprayed with a compound, including zirconia, to inhibit over-wetting past the steel surface of the part. The parts are then positioned in an appropriate device, heated to an elevated temperature for infiltration of the metal, and the furnace is cooled again. The cooling process in each instance is lengthy and time consuming. This is a disadvantage in the process adding costs, delay, and unnecessary handling.
The zirconia has been found to function as a barrier to the infiltration of the metal infiltrant. This is accomplished because of the different surface energies between the zirconia and the metal infiltrant so that the zirconia prevents the over-wetting or bleed-through of the metal infiltrant beyond the steel surfaces of the parts being formed.
An improved refractory furnace process has been developed that combines the sintering and infiltration steps so that the green parts are sprayed with an improved surface enhancer prior to being positioned in the appropriate apparatus for binder burnout, sintering and infiltration in a single step with accelerated cooling.
This process is described in U.S. Pat. No. 5,989,476. However, the improved process creates a problem of increased infiltrant bleeding through the coating and leaving the finished steel parts with an undesirable surface appearance, including discoloration with a gold-type appearance and cracking or wrinkling. The gold discoloration is attributed to the flaking off of the zirconia from the article during the binder burnout phase of the process. Additionally, there are problems encountered with the presence of carbon residue that is scavenged from the binder gases into the infiltrant. The sequestered carbon is subsequently released in the process after the infiltrant melts. It thereby forms a slag on the surface that inhibits the flow of the infiltrant into the metal part, resulting in infiltrant starvation in the finished part when the carbonaceous slag retains the infiltrant in the reservoir apparatus during the process.
Articles formed by the selective laser sintering process utilizing metal powder or metal/ceramic powder are also subject to bleed through problems. A suitable process and apparatus for performing selective laser sintering is described in U.S. Pat. Nos. 4,863,538 and 5,252,264.
These problems are solved in the use of the improved enhancer formulation of the present invention.
It is an aspect of the present invention that an improved polymer inhibitor is added to the surface enhancer solution that is applied to the green part in a furnace process to produce an infiltrated metal or metal/ceramic composite article which eliminates excess metal infiltrant-bleed-through in the finished part.
It is another aspect of the present invention that the improved polymer inhibitor is a polymer with an optimum formulation of between about two to four percent concentration by weight of the carrier solvent in the total enhancer formulation.
It is a feature of the present invention that the polymer inhibitor added to the enhancer is a poly(styrene-co-maleic acid), partial sec-butyl/methyl ester that is combined with the suspension of zirconia particles in a solvent.
It is another feature of the present invention that a zirconate coupling agent is used with the enhancer formulation to inhibit infiltrant-bleed-through and reduce surface cracking and wrinkling in the finished part.
It is still another feature of the present invention that the preferred coupling agent is a zirconate that is added to the zirconia inhibitor prior to the addition of the polymer.
It is an advantage of the present invention that the improved enhancer reduces metal infiltrant-bleed-through in the final finished part.
It is another advantage of the present invention that the improved enhancer reduces undesirable cracked and wrinkled surfaces in the finished part.
These and other aspects, features and advantages are obtained in the present invention by use of an improved enhancer with a solvent, a polymer, a